1. Field of the Invention
The present invention relates to an electrophotographic apparatus and an electrophotographic cartridge. The electrophotographic apparatus and the electrophotographic cartridge of the present invention are applied to a photocopier, facsimile machine, laser printer or direct digital platesetter and the like.
2. Description of the Related Art
Although the electrophotographic photoconductors used in electrophotographic apparatuses applied to photocopiers, laser printers and the like formerly consisted primarily of inorganic photoconductors such as selenium, zinc oxide and cadmium sulfate, organic photoconductors (OPC), which are more advantageous than inorganic photoconductors in terms of reducing the burden on the global environment, reducing costs and allowing greater freedom of design, are currently used at a rate that is rapidly approaching 100% of the total production volume of electrophotographic photoconductors.
In recent years, accompanying greater importance being placed on production in consideration of protecting the global environment, the role of photoconductors is required to change from that of consumable supplies (disposable products) to mechanical components. In order to achieve this, it is necessary for photoconductors to offer a longer service life, and in order to respond to this need, it has become common to add a protective layer onto the photosensitive layer.
In addition, the use of polymerization toners (spherical toners) is becoming increasingly common for the toners for development used in electrophotography for reasons of improved reduction in the burden placed on the environment during toner production as well as being advantageous in terms of achieving high image quality.
This polymerization toner (spherical toner) is a spherically shaped toner that is free of corners, and is produced by chemical production processes such as suspension polymerization, emulsion aggregation polymerization, ester elongation polymerization or dissolution suspension. Polymerization toners have different shapes depending on the production process, and the polymerization toner used in image forming apparatuses has a shape that is slightly distorted from being perfectly round. Typical characteristic values consist of an average circularity of 0.95 to 0.99, values of 110 to 140 for shape factors SF-1 and SF-2, and although polymerization toners having a particle size distribution of ±0.5 μm can be produced, those having a particle size distribution of ±1 μm to 2 μm are typically used. Those having an average circularity of 1.0 and value of 100 for shape factors SF-1 and SF-2 are perfectly round.
Since polymerization toners have a uniform shape, the charge they possess can be made to be uniform comparatively easily. In addition, wax and the like can also be added easily (5% to 10%). Thus, since there is hardly any overflow from latent electrostatic images, polymerization toners have satisfactory developing ability, superior sharpness, resolution and contrast and satisfactory transfer efficiency. In addition, they also have numerous advantages such as oilless transfer.
On the other hand, this type of toner is known to be associated with difficulties in cleaning, and numerous countermeasures for this have been proposed in the patent literature.
In general, in order to ensure adequate cleanability for these toners, it is desirable that the surface of the photoconductor has a low friction coefficient and that the friction coefficient be maintained during repeated use.
For example, cleaning performance of a polymerization toner is known to be secured by lowering the friction coefficient of a photoconductor surface by coating the surface thereof with a lubricant such as zinc stearate (to simply be referred to as lowering the surface energy) (Nobuo Hyakutake, Akihisa Maruyama, Satoru Shigesaki, Sachie Okuyama: Japan Hardcopy Fall Meeting, 24-27, 2001).
However, when a lubricant is externally added and supplied to the surface of a photoconductor, the lubricant is mixed into the toner to be recycled, resulting in degradation of the toner. In addition, if a lubricant that has degraded in the charging step and so forth remains on the surface of the photoconductor, there are problems such as occurrence of image flow and poor performance of the cleaning blade.
Another typically known method involves the containing of a lubricant such as a silicone compound, fluorine resin fine particles or fatty acid ester in the outermost surface of a photoconductor. A means for containing fluorine atom-containing resin fine particles in the outermost layer of a photoconductor is proposed for the cleanability of polymerization toners in particular (for example, Japanese Patent Application Laid-Open (JP-A) Nos. 11-218953 and 11-272003).
The energy on the surface of a photoconductor is effectively lowered by containing fluorine atom-containing resin fine particles. However, the friction coefficient on the photoconductor surface is increased by endurance, and it is difficult to maintain the initial surface friction coefficient only by containing fluorine atom-containing resin fine particles. In this case, the friction coefficient of the photoconductor surface is increased soon after started to use, and cleanability becomes insufficient. Thus, it is necessary to replace the photoconductor.
In addition, it is necessary to contain fluorine atom-containing resin fine particles at a prescribed concentration or more in order to reduce the friction coefficient of the photoconductor surface, in this case, the film strength is weakened as is described in the paragraph [0001] of JP-A No. 07-13381 and the paragraph [0026] of JP-A No. 10-142816. In addition, there are many cases in which wear resistance of the photoconductor is decreased by containing fluorine atom-containing resin fine particles even if it is prepared to the contained amounts defined in the patent literature.
As current needs, it is determined that an electrophotographic photoconductor maintaining both high wear resistance and a low surface friction coefficient is demanded. Naturally, high sensitivity and stable characteristics with respect to environmental changes are also essential. However, a method of satisfying these demands has yet to be found.
In the most common electrophotographic apparatuses in use today, the cleaning blade accounts for the majority of the cleaning performance of polymerization toners. Thus, it is advisable to enhance the cleaning performance of the cleaning blade in order to enhance the cleanability of the polymerization toner.
The cleaning failure of polymerization toner is thought to be caused by an imbalance in the contact conditions between the cleaning blade and the photoconductor (for example, JP-A No. 2001-242758) and the toner itself getting beneath the portion contacted by the cleaning blade (for example, JP-A No. 2002-189312). If the frictional resistance between the cleaning blade and photoconductor is high, the edge of the blade is completely dragged in the driving direction of the photoconductor. Moreover, when the photoconductor and blade edge unevenly contact and cause chatter, the blade edge may be twisted or lifted and then a gap is formed. The toner moves into the gap, rolling into the gap, and then escapes because of its spherical shape. If the toner escapes in this manner, the toner continues to successively enter the gap and cleaning failure occurs.
In order to retain the toner with the cleaning blade, it is important to set suitable conditions for the contacting the cleaning blade to the photoconductor while also stabilizing those conditions.
The contact conditions are made unsuitable by the following factors.
(1) Large Frictional Resistance Between Photoconductor and Cleaning Blade
If the frictional resistance between the cleaning blade and photoconductor is excessively large, the entire contact site of the cleaning blade has difficulty in uniformly contacting the driving photoconductor. At this time, the edge of the cleaning blade is dragged in the driving direction of the photoconductor and contact pressure is no longer applied uniformly, thereby resulting in distortion and twisting or causing vibrations such as chattering. Consequently, toner escapes.
(2) Chipping of the Blade Edge
If the edge of the cleaning blade is chipped, the cleaning failure in the form of lines occurs, and the width and density of the lines are different depending on the size of the missing portions. Even if there are some missing portions in the blade edge, the contact pressure is still effective and the blade edge becomes flattened so as to cover the missing portions. Thus, few problems occur. The excessively large missing portions cannot be adequately prevented, and causes the cleaning failure in lines, which may eventually lead to cleaning failure.
Although there are several causes for the occurrence of chipping of the edge of the cleaning blade, particular caution is required with respect to accumulation of fatigue in the blade edge caused by excessive contact pressure of the cleaning blade or a large friction coefficient of the photoconductor, and the adhesion of substances such as the carrier (magnetic particles) or toner film to the surface of the photoconductor continuously contacting with the blade edge.
(3) Unsuitable Blade Contact Pressure
As the set value of the blade contact pressure is higher, a gap is difficultly formed between the photoconductor and edge of the cleaning blade, thereby improving cleaning failure. However, the photoconductor having a high friction coefficient may cause inversion or locking of the blade, deformation of the blade, the occurrence of sticking and slipping phenomena, increased wear of the photoconductor, scratches and accelerated wear and deformation of the cleaning blade. Thus, the cleaning failure easily occurs.
In addition, when the surface area of the photoconductor contact site of the cleaning blade is increased, the maximum value of the pressure distribution of the contact portion is decreased. As a result, the force that enables a toner to be retained by the cleaning blade may be decreased.
On the other hand, when the contact pressure of the cleaning blade is low, the load on the photoconductor and cleaning blade is decreased thereby increasing mechanical durability, but the penetration of toner beneath the cleaning blade. In the case where the photoconductor and cleaning blade are not in a good status, the cleaning failure more easily occurs.
As has been described above, when the pressing pressure (linear pressure) of the cleaning blade on the photoconductor is increased, the cleaning failure may be improved, but, the wear of the blade edge may be accelerated and the force used to retain the toner is easily decreased. Thus, a strategy and photoconductor capable of inhibiting the wear of the blade edge are required.
(4) Large Surface Roughness of the Photoconductor
When the surface roughness of the photoconductor is excessively large, the adherence of the cleaning blade to the photoconductor surface becomes inadequate. The chips formed in the blade edge, or the vibration of the blade edge may be more easily form a gap.
For example, when a toner is embedded in indentations in the photoconductor surface, which forms an escape path where the cleaning blade passes over the toner.
A toner and paper dust adhesion and damage to the photoconductor surface may cause increased surface roughness of the photoconductor.
Reducing the friction coefficient of the photoconductor surface and maintaining a low friction coefficient are advantageous for resolving these problems associated with contact conditions. This is because pulling of the cleaning blade in the driving direction of the photoconductor is alleviated by reducing the frictional force between the photoconductor and the cleaning blade. In addition, the inventors of the present invention confirmed experimentally that the depth to which chips form in the blade edge can be allowed to about 70 μm when the contact pressure of the cleaning blade is set to 22 g/cm to 25 g/cm in an electrophotographic apparatus using a polymerization toner, and a photoconductor having a friction coefficient of about 0.2 to 0.3 is used. Thus, in case of chipping of the edge of the cleaning blade, the robustness of the cleaning blade on cleaning performance is enhanced by decreasing the friction coefficient (disclosed in JP-A No. 2004-279518).
In addition, it is also possible to enhance to a certain extent the toner cleanability of an apparatus by increasing the contact pressure of the cleaning blade. However, since the load increase accelerates wear of the cleaning blade edge, thus, this is not suited for extending the service life of the apparatus. The increase of the contact pressure also acts advantageously in photoconductors having a low friction since the wear rate is inhibited. In addition, the excellent cleanability of the polymerization toner can be acquired depending on the selection of the photoconductor, provided that surface roughness of the photoconductor can be reduced.
Methods for reducing the friction of the surfaces of photoconductors have long been studied, and the method of containing a lubricant in the form of a silicone oil or fluorine resin fine particles into the surface layer of a photoconductor is widely known. A lubricant is contained in a condition of a high concentration in the surface layer of the photoconductor so as to maintain the low friction (JP-A No. 2005-62830).
In addition, a photoconductor has been proposed that realizes both wear resistance and electrical characteristics of the surface layer by containing resin fine particles in a crosslinked polymer layer obtained by curing a radically polymerizable monomer having three or more functional groups not having a charge transporting structure and a radically polymerizable compound having a charge transporting structure (JP-A Nos. 2005-227742 and 2005-208112).
However, by containing a large amount of a lubricant in the photoconductor surface layer, the enhancement of wear resistance is limited. Thus, it has been desired that a low friction of the photoconductor can be maintained without containing a lubricant.
As a photoconductor capable of maintaining a low friction without containing a lubricant, it has been proposed that an electrophotographic photoconductor having a crosslinked layer obtained by curing a radically polymerizable monomer having three or more functional groups but not having a charge transporting structure, a radically polymerizable compound having a charge transporting structure and a single functional group, and a reactive silicone compound having a radically polymerizable functional group and a dimethylsiloxane structure as a repeating unit (JP-A No. 2005-115353). In this electrophotographic photoconductor, a silicone oil is chemically bonded to a three-dimensional crosslinked resin matrix so as to prevent in advance any loss of lubricant caused by bleedout. However, the electrophotographic photoconductor has the problem of toner escaping during cleaning when using a toner having high circularity such as polymerization toner.